Browsing by Subject "Fracture"
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Item A comprehensive study of nano-scale grain boundary channels in fracture cements using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy(2017-05) Wright, Erick M.; Eichhubl, Peter; Fisher, William; Behr, WhitneyNatural fractures in shale reservoirs are frequently filled with mineral cement that lack any residual fracture porosity visible under the petrographic microscope and are generally interpreted to be impermeable to fluid flow. Scanning electron microscopy of calcite, dolomite, quartz, and barite fracture cements from a variety of shale and low-permeability sandstone formations, prepared using broad ion beam milling, provides evidence of open to partially healed elongate pores that are on the order of hundreds of nanometers in aperture. In calcite, dolomite, quartz, and barite fracture cements, these pores have apertures of about 10-400 nm. In quartz fracture cements that have experienced low grade metamorphic temperatures in excess of 250°C, they are up to 600 nm. These pores have been previously overlooked because traditional thin sectioning and polishing destroys sub-micron details of the fracture cement pore structure. Ion milling preserves these details. Electron backscatter diffraction shows that these pores occur along most grain boundaries within the blocky or columnar fracture cement. Mineral cement crystal faces are rarely faceted on the nano-to-micrometer scale, but contain varying degrees of roughness with distinct morphologies. Pores tend to increase in aperture with increasing maximum formation temperature, indicating dissolution-precipitation kinetics strongly influences grain boundary structure. Stress relaxation from formation exhumation may also favor wider channel apertures, evidenced by a trend of wider apertures with increased distance of exhumation. HRTEM analysis shows different crystallographic domains with possible amorphous regions bridging across grain boundary channels, demonstrating that these channels are complex structures, contrasting the conventional view of diagenetic cement grain boundaries as simple crystallographic dislocations or discontinuities. I propose a model of dissolution-precipitation along grain boundaries that preserves NGBC ubiquitously in carbonate and quartz fracture cements that have experienced diagenetic and low-grade metamorphic conditions. While partially healed, these pores are frequently well connected and have strong implications for understanding flow through matrix cements of low permeability reservoirs. They may act as conduits for fluid flow along and across fully cemented natural fractures. When their effects are considered, they may universally increase estimates of formation permeability of low permeability formations containing cemented fractures.Item Analysis of the effects of carbonate mounds on associated stratal geometry and fracture development, Sacramento Mountains, New Mexico, USA(2016-12) Tinker, Nathan Scott; Janson, Xavier; Zahm, Christopher Kent; Kerans, Charles; Fisher, William LThe objective of this research is an integrated structural‐stratigraphic analysis of compaction‐related fracturing in carbonate mounds and associated cover strata. The influence of early-cemented carbonate mounds on subsequent sediment deposition (such as creation of hard substrates and topographic relief) is relatively well-understood. The effect of early-cemented carbonate mounds during burial, however, has not been studied in detail. Early marine cementation of mounds enhances mechanical rigidity, which reduces mound compaction during burial as compared to less-resistant sediments surrounding and overlying the mound. This rigidity difference facilitates differential compaction of sediments overlying the mound, which are warped over the inflection point created by the mound topography. This study hypothesizes that there is a measurable increase in fracture intensity associated with differential compaction above early-lithified carbonate mounds. Thus, this work analyzes and quantifies the effects of differential compaction on stratal geometry, mechanical stratigraphy, and fracture development in Mississippian strata overlying carbonate mounds which are well-exposed in the Sacramento Mountains in southeast New Mexico. Methods employed in this study are drawn from structural geology, sedimentology, petrography, and remote sensing in an effort to adequately determine facies, examine fracture characteristics (e.g. size, orientation, and intensity), and to better understand which process(es) most directly control those characteristics (e.g. host rock facies type, diagenesis, bed thickness, mound proximity, mound size). Innovative methods of outcrop characterization such as high-resolution gigapan photography and unmanned aerial vehicle (UAV) photography were combined with photogrammetric techniques to create photo-realistic 3D outcrop models. The resulting models enabled a cost-effective, more detailed, less-distorted, and more comprehensive interpretation compared to previous methods, and improved understanding of the relationship between stratigraphy, rock mechanical evolution, and structural deformation in carbonate mound systems. Field work documented facies, stratal geometries, folds, faults, and fracture sets which validated observations and characterizations made using high-resolution field photographs and 3D outcrop models. Results of this work show that paleotopographic relief which has been early lithified (in this instance, Mississippian carbonate mounds) directly controls fracture development and overlying stratal geometry, in that there is a significant increase in tension fracture (mode 1) intensity above pre-existing rigid structures and over-steepening of bed dips beyond an expected and reasonable angle of repose. Additionally, this work outlines a multi-stage tectonostratigraphic sequence of the development of the stratigraphically complex Teepee Mound assemblage based on field observations of facies, fractures, stratal geometries, and diagenetic effects (e.g. cementation, compaction, and chertification), which includes new evidence of late-Mississippian tectonic compression. This result emphasizes the importance of understanding both syndepositional and post-depositional processes in outcrop characterization. Specifically, syndepositional processes establish the original mechanical stratigraphy and control the formation and propagation of early mechanical discontinuities, which in turn set up the fabric of weaknesses preferentially utilized by later fracture development. Post-depositional mechanical and diagenetic processes alter mechanical stratigraphy and rock brittleness, and thus influence fracture propagation through time.Item Characterization and modeling of mixed-mode I+III fracture in brittle materials(2015-12) Pham, Khai Hong; Ravi-Chandar, K.; Landis, Chad M; Liechti, Kenneth M; Mear, Mark E; Marder, Michael PMixed-mode I+III fracture in brittle materials presents spectacular, scale-independent pattern formation in nature and engineering applications; and it is one of the last remaining puzzles in linear elastic fracture mechanics. This problem has received much attention in the literature over the past few decades both from experiments and analysis, but there are still open challenges that remain. Specifically, the existence of a threshold ratio of mode III to mode I loading below which fragmentation of the crack front (formation of daughter cracks) does not occur and the length scale associated with the spacing of the fragments when they do occur are still under debate. The continued growth of cracks under remote mode I + III loading is also of interest; it is observed that in some cases the fragmented cracks coalesce, while in others they maintain their independent development. We approach this problem through carefully designed experiments to examine the physical aspects of crack initiation and growth. This is then explored further through numerical simulations of the stress state that explore the influence of perturbations on the formation of daughter cracks. We show that a parent crack subjected to combined modes I+III loading exhibits fragmentation of the crack front into daughter cracks without any threshold. The distance between the daughter cracks is dictated by the length scale corresponding to the decay of the elastic field; this decay depends on the characteristic dimension of the parent crack from which the daughter cracks are nucleated. As the daughter cracks continue growing, they coarsen in spacing also through elastic shielding. As the daughter cracks grow farther, the parent crack, pinned at the original position, experiences increased stress intensity factor and the bridging regions begin to crack and the parent crack front advances towards the daughter cracks. This establishes a steady state condition for the system of parent crack with equally spaced daughter cracks to continue growing together. Finally, direct numerical simulation of crack initiation and growth is explored using a phase-field model. The model is first validated for in-plane modes I + II through comparison to experiments, and then used to explore combined modes I + III in order to study the above mechanism of mixed-mode I + III crack growth.Item Constraining fracture permeability by characterizing fracture surface roughness(2010-12) Al-Johar, Mishal Mansour; Sharp, John Malcolm, 1944-; Ketcham, Richard A.; Cardenas, Meinhard B.Open and connected fractures, where present, control fluid flow and dominate solute transport. Flow through fractures has major implications for water resource management, underground waste repositories, contaminant remediation, and hydrocarbon exploitation. Complex fracture morphology makes it difficult to quantify and predict flow and transport accurately. The difficulty in usefully describing the complex morphology of a real fracture from a small 3-D volume or 2-D profile sample remains unresolved. Furthermore, even when complex fracture morphology is measured across three-dimensions, accurate prediction of discharge remains difficult. High resolution x-ray computed tomography (HXRCT) data collected for over 20 rock surfaces and fractures provide a useful dataset to study fracture morphology across scales of several orders of magnitude. Samples include fractured rock of varying lithology, including sandstone, volcanic tuffs and crystalline igneous and metamorphic rocks. Results suggest that the influence of grain size on surface roughness is not readily apparent due to other competing variables such as mechanics, skins and coatings, and weathering and erosion. Flow tests of HXRCT-scanned fractures provide real discharge data allowing the hydraulic aperture to be directly measured. Scale-invariant descriptions of surface roughness can produce constrained estimates of aperture variability and possibly yield better predictions of fluid flow through fractures. Often, a distinction is not made between the apparent and true fracture apertures for rough fractures measured on a 2-D topographic grid. I compare a variety of local aperture measurements, including the apparent aperture, two-dimensional circular tangential aperture, and three-dimensional spherical tangential aperture. The mechanical aperture, the arithmetic mean of the apparent local aperture, is always the largest aperture. The other aperture metrics vary in their ranking, but remain similar. Results suggest that it may not be necessary to differentiate between the apparent and true apertures. Rock fracture aperture is the predominant control on permeability, and surface roughness controls fracture aperture. A variety of surface roughness characterizations using statistical and fractal methods are compared. A combination of the root-mean-square roughness and the surface-to-footprint ratio are found to be the most useful descriptors of rock fracture roughness. Mated fracture surfaces are observed to have nearly identical characterizations of fracture surface roughness, suggesting that rock fractures can be sampled by using only one surface, resulting in a significantly easier sampling requirement. For mated fractures that have at least one point in contact, a maximum potential aperture can be constrained by reflecting and translating a single surface. The maximized aperture has a nearly perfect correlation with the RMS roughness of the surface. These results may allow better predictions of fracture permeability thereby providing a better understanding of subsurface fracture flow for applications to contaminant remediation and water and hydrocarbon management. Further research must address upscaling fracture morphology from hand samples to outcrops and characterizing entire fracture networks from samples of single fractures.Item Diatremes(2009-03) Barker, Daniel S.Item Differential compaction fractures in carbonate mound complexes : pioneering numerical models applied to outcrops and subsurface reservoirs(2018-08) Alzayer, Yaser Abdullah; Kerans, C. (Charles), 1954-; Zahm, Christopher Kent; Janson, Xavier; Sharp, John M; Steel, RonaldDifferential compaction is thought to be a primary driver for syndepositional fracture development in carbonate platforms. Outcrop and subsurface observations of syndepositional fractures in carbonate mound complexes and platforms cannot be used to directly identify the mechanism or controlling factors behind their formation, because these observations represents the end state of potentially long and complex stress and diagenetic history. The limitations of outcrop observations are overcome by using a finite-element and combined finite-discrete forward models to simulate differential compaction and subsequent fracture development in carbonate mound complexes. Differential compaction deformation is modeled at the mound scale (tens of meters) and at an isolated platform-scale (kilometers). Numerical models are used to (1) quantify amount of differential subsidence required to develop fractures, (2) predict areas susceptible to fracture development, and (3) identify the most critical factors controlling differential compaction fracturing. 2D and 3D models are constructed based on classic outcrops of Late Pennsylvanian carbonate mounds in the Sacramento Mountains and age-equivalent Canyon and Cisco formations in the Midland Basin, West Texas. Modeling results are consistent with fracture observations in outcrops and the subsurface. Geometry of lithified antecedent topography and the overlying strata controls the location of differential compaction fractures. Fractures develop in strata overlying antecedent topography in transitional crest-to-off-mound/platform areas. Another location for fracture development corresponds to strata overlying the mound/platform slope-to-off-mound/basinal setting transition. Modeling results demonstrate that only a minor amount (cm -10s cm scale) of differential subsidence is required to develop fractures in early lithified carbonates. This suggests that differential compaction fractures in carbonate systems may be generally underestimated. Fracture intensity is found to be proportional to the amount of differential subsidence. A greater control on fracture intensity is the bedding contact nature. Fracture development in strata with bedding contacts that are resistant to layer-parallel slip display almost double the fracture intensity of strata with contacts favoring slip. Layer-parallel slip is concluded to be a major mechanism for dissipating stress during compaction-driven folding. The process-based modeling approach applied by this work provides fundamental understanding of differential compaction fracture development in carbonate mound complexes, which is valuable to prediction of fractures in subsurface reservoirs.Item Dikes(2009-03) Barker, Daniel S.Item Dynamic response of metal-polymer bilayers subjected to blast loading(2012-12) Albrecht, Aaron Berkeley; Ravi-Chandar, K.; Landis, Chad; Liechti, Kenneth; Mear, Mark; Marder, MichaelThe use of compliant coatings, in particular polyurea, for improved blast protection of structures has been reported recently in the literature. The goal of this research is to develop a comprehensive understanding of the reasons for improved performance of coated structures through experimentation and correlation with simulation. The different factors influencing the response of an elastomer coated ductile metal subjected to a blast load have been examined and quantified. First, dynamic strain localization in the metal is a precursor to ductile failure; this was characterized for the metal of interest with and without the polymer coating. Experiments with the expanding ring/tube and experiments have demonstrated that for Al 6061-O and Al 3003-H14, the localization strain is unaffected by both deformation rate and the polymer coating; however, two important effects of the coating have been explored. First the additional mass of the coating provides an inertial resistance. Second, the flow resistance of the polymer provides continued dissipation of energy even after the metal has yielded potentially preventing failure in the metal, or at least containing fragments. These effects were examined for two different types of polymers – polyurea, an elastomer and polycarbonate, a thermoplastic shear yielding polymer. It is shown that these two effects can be used to tailor the coating to optimize blast protection of the bilayer system. In order to take advantage of this optimization, the constitutive behavior of the elastomer coating must be determined at strain rates and loading conditions that are experienced in the blast loading; these strain rates are in the range of 1000 to 10,000 per second. This has been accomplished through a hybrid method that combines measurements with numerical simulations to extract the constitutive response of the material. The strain rate dependent behavior of polyurea for rates in the range of 800-8000 per second has been determined by measuring the spatio-temporal evolution of the particle velocity and strain in a thin strip subjected to high speed impact loading that generates uniaxial stress conditions and comparing this with numerical simulations of the one-dimensional problem using the method of characteristics. A similar scheme to track the particle velocity and strain during the axisymmetric deformation of a membrane subjected to high speed loading has also been developed; this requires two projections of the deformation to be obtained in order to facilitate the measurement of axial and kink waves in the membrane. The finite volume method is adapted for simulations of these dynamic uniaxial and axisymmetric problems with a view towards simulating shock waves that are expected to form in some loading conditions. The hybrid method is used once again to characterize the constitutive response. The axisymmetric experiments have demonstrated the inability of the uniaxial models for both polyisoprene rubber and polyurea to completely capture their behavior during a more complex loading, and left a need for further work on characterizing the dynamic constitutive response of these polymers.Item Effect of electro-mechanical loading in metallic conductors(2010-12) Gallo, Federico Guido; Ravi-Chandar, K.; Mear, Mark E.; Satapathy, Sikhanda S.; Liechti, Kenneth M.; Landis, Chad M.The development of high powered electro-magnetic devices has generated interest in the effect of combined electromagnetic and mechanical loading of such structures. Materials used in high-current applications – aluminum alloys and copper – are subjected to heat pulses of short duration (in the range of a few hundred microseconds to a few milliseconds); immediately following or along with such heat pulses, these materials are also subjected to large mechanical forces. In previous work reported in the literature, ejection of material from the vicinity of preexisting defects such as cracks, notches or discontinuities have been observed resulting from short-duration high-intensity current pulses; after a series of pulses, permanent deformation and weakening of intact material has also been reported. But a lack of complete understanding of the effects of short duration current pulses hinders the assessment of the reliability of such conductors in high energy applications. Therefore, an investigation was undertaken to examine the behavior of electromagnetically and mechanically loaded conductors. This work investigates the effects of short-duration, high-current-density pulses in combination with viii mechanical loading. The aim is to develop a theoretical model to describe the resulting mechanical response. The model is to provide a characterization of the possible effects of thermally-induced plastic strains on metals loaded beyond or just below their yield strength or below the critical stress intensity factor. In the experiments reported here, two types of specimens, undamaged and damaged, were subjected to combined electromechanical loads. Undamaged specimens were used to observe thermally-induced plastic strains - strains not caused by an increase in mechanical loading, but rather resulting from the reduction of yield strength and post-yield stiffness due to the increase in temperature. The experiments were conducted such that it would be possible to develop a model that would conclusively account for the observed material behavior. The second sets of specimens were weakened a priori by the introduction of a crack in order to study the influence of such crack-like defects on the electrical and mechanical fields, and to produce a safe design envelope with respect to the loading conditions. Failure was found to occur due to melting triggered by joule heating; a quantitative criterion based on current concentration and heat accumulation near the crack tip has been developed based on these experimental results.Item Effect of in-plane voiding on the fracture behavior of laser sintered polyamide(2011-12) Leigh, David Keith; Bourell, David Lee; Beaman, Joseph J.Laser Sintering, a method of additive manufacturing, is used in the production of concept models, functional prototypes, and end-use production parts. As the technology has transitioned from a product development tool to an accepted production technique, functional qualities have become increasingly important. Tension properties reported for popular polyamide sintering materials are comparable to the molded properties with the exception of elongation. Reported strains for laser sintered polyamide are in the 15-30% range with 200-400% strains reported for molding. (CES Edupack n.d.) The primary contributors to poor mechanical properties in polyamide materials used during Selective Laser Sintering® are studied. Methods to quantify decreased mechanical properties are compared against each other and against mechanical properties of components fabricated using multiple process parameters. Of primary interest are Ultimate Tensile Strength (UTS) and Elongation at Break (EOB) of tensile specimens fabricated under conditions that produce varying degrees of ductile and brittle fracture.Item Effect of moisture on mixed-mode TSR on a glass/epoxy interface(2017-12) Ferreira Vieira de Mattos, Daniel; Liechti, K. M.; Huang, Rui; Rodin, Gregory J; Ravi-Chandar, Krishnaswa; Bonnecaze, Roger TThe understanding of interfacial failure in adhesively bonded structures is important for several sectors including transportation and infrastructure. This problem has motivated studies for several decades. Adhesives are polymeric and, as such, present time, temperature, strain rate and moisture dependence. The effect of moisture on interfacial adhesion and fracture is still an open problem and demands a deep multi-disciplinary study considering nonlinear viscoelasticity, fracture mechanics, diffusion, chemistry and surface science. This is justified through the mechanisms through which moisture can affect interfacial adhesion. The presence of moisture can degrade the interface integrity. The absorbed moisture also modifies the mechanical properties of the bulk adhesive as well as its interactions with substrates, which introduces changes in the response of the adhesively bonded structure as it is subjected to an external load. An additional complication for interfacial cracks constrained to grow along the interface is that crack growth is governed by the tensile and shear stresses at the interface as well as the interfacial interactions (adhesion energy, strength and range) embodied in traction separation relations and giving rise to the term mixed-mode fracture. This research investigates the effect of moisture on interfacial fracture for different mode-mixes. The content is developed in four parts. First, the adhesive is experimentally characterized via the following tests: mechanical loading, water diffusion, thermal and hygral expansion. These results introduce the second part: a nonlinear viscoelastic model calibrated considering all those measured properties. This model captures the effect of time, temperature, strain rate and moisture on the mechanical behavior of the adhesive. The third part deals with the fracture behavior of a glass/epoxy interface over a range of mode-mixes and moisture conditions. This is complemented by analyses including optical profile measurements of the fractured surfaces and extraction of traction and separation relations and toughness. Finally, a significant emphasis was placed on the numerical analysis which was required for each of the three components outlined above.Item The enriched Galerkin method for linear elasticity and phase field fracture propagation(2015-12) Mital, Prashant; Wheeler, Mary F. (Mary Fanett); Wick, ThomasThis thesis focuses on the application of the discontinuous Galerkin (DG) and enriched Galerkin (EG) methods to the problems of linear elasticity and phase field fracture propagation. The use of traditional and popular continuous Galerkin method (CG) for linear elasticity has posed some challenges. For example, nonphysical stress oscillations often occur in CG solutions for linearly elastic, nearly incompressible materials. Furthermore, CG solutions produce discontinuous stresses at the finite element boundaries which need to be post-processed. Based on the success of the DG methods in solving these challenges, we attempt resolution of the same problems with the yet untested EG method. For phase field fracture propagation, the CG method has been ubiquitously used in the literature. Since the phase field displacement solution is essentially discontinuous across the crack, we hypothesize that the discontinuous DG and EG methods could offer some advantages when applied to the fracture problem. We then perform a comparative analysis of CG, DG and EG applied to the phase field equations to determine if this is indeed the case. We begin by applying a family of DG and EG methods, including Nonsymmetric Interior Penalty Galerkin (NIPG), Symmetric Interior Penalty Galerkin (SIPG), and Incomplete Interior Penalty Galerkin (IIPG) to 2D linear elasticity problems. It is shown that the EG methods are simple and robust for dealing with linear elasticity. They are also shown to converge at the same rates as the corresponding DG methods. A detailed comparison of the performance of NIPG, IIPG, and SIPG is also made. We then propose a novel monolithic scheme with an augmented-Lagrangian method for phase field fracture propagation. We apply CG, DG and EG methods to the scheme and establish convergence in space and time through numerical studies. It is shown that the Newton method used for solving the system of nonlinear equations converges faster for DG and EG than it does for CG.Item Experimental investigation of ASR/DEF-induced reinforcing bar fracture(2011-12) Webb, Zachary David; Bayrak, Oguzhan, 1969-; Zhu, Jinying; Jirsa, James O.Numerous cases of premature concrete deterioration due to alkali-silica reaction and/or delayed ettringite formation have developed within highway infrastructure in the state of Texas over the past two decades. Although experimental research and in-situ load testing on an international scale has indicated that moderate levels of deterioration are unlikely to pose a threat to structural safety, the discovery of reinforcing bar fracture in Japan due to ASR-related expansion has called into question the integrity of heavily damaged structures. A two-part experimental program was conducted at The University of Texas at Austin relating to ASR/DEF-induced reinforcing bar fracture. Work conducted under TxDOT Project 0-6491 included the fabrication and monitoring of four concrete specimens. Methods were employed to simulate a fracture of the transverse reinforcement within the time frame of the study and the applicability of various NDT monitoring techniques to detect bar fracture was evaluated. Furthermore, a number of reinforcing bar samples were tested and analyzed to investigate (1) the development of reinforcing bar cracking due to the bending operation and (2) the progression of cracks after application of an expansive opening force on bars with 90° bends. Research findings and conclusions form a preliminary assessment on the potential for reinforcing bar fracture within affected infrastructure in Texas.Item Exploring the physics of methane-foam generation for gas mobility control in high-temperature, proppant-fractured reservoirs(2022-12-05) Parekh, Aashish Trilok; Nguyen, Quoc P.; Katiyar, AmitGas EOR through HnP is an increasingly important method of recovering additional oil from fracture-stimulated reservoirs. HnP productivity, however, is hampered by well interference and fracture channeling leading to early gas breakthrough and poor areal sweep efficiency. To mitigate these issues and improve conformance control, foam-generating surfactants have been developed as a method of reducing mobility of the injected gas phase and increasing oil recovery. The experiments outlined in this work investigated foam generation and propagation by an anionic and amphoteric surfactant blend in high-temperature, high-pressure, high-permeability, and high-shear conditions meant to simulate the actual environment of a proppant-filled fracture. Bulk foam tests confirmed the aqueous stability and foaming viability of the surfactant at the proposed temperature, pressure, and salinity. Through several series of floods co-injecting methane gas and the surfactant solution through a proppant-pack at residual oil, the effects of several injection parameters on apparent foam viscosity were investigated. The surfactant exhibited an unusually high transition foam quality at about or greater than 95% as well as expected shear-thinning behavior. When analyzing the effect of pressure on foam generation, an important finding was observed in that foam viscosity linearly decreased with increasing pressure. Another flood series was conducted in an oil free proppant pack and showed that residual oil had no effect on the apparent foam viscosity, meaning oil swelling at higher pressures could not be the reason the inversely linear pressure trend was seen. An additional flood series with nitrogen as the injection gas was completed to see if the hydrophobic attraction between the methane and surfactant tail was responsible for the observed pressure trend, but, despite an increase in apparent foam viscosity, the pressure trend persisted even with nitrogen. Though a conclusion could not be made on the cause of the pressure dependency, previous tests confirmed that the trend is not the result of a system artifact and supported that it must be due to the effect of pressure on surface properties of foam film stabilized by this particular surfactant formulation. Additional tests focused on the effect of pressure on surfactant adsorption and micellar size could prove fruitful in finding an answer.Item Extension of phase-field modeling to fatigue and large structures(2022-05-06) Lo, Yu-Sheng, Ph. D.; Landis, Chad M.; Mear, Mark E; Ravi-Chandar, Krishnaswa; Huang, Rui; Foster, John TThe phase-field approach to fracture naturally captures the emergence of complex crack patterns and behaviors, including crack nucleation, turning, branching, and intersection in both two and three dimensions without any additional constitutive assumptions for these behaviors. What follows will present the study of fatigue crack growth using the phase-field approach. A crack growth viscosity parameter is introduced into the standard phase-field model for brittle fracture to account for rate- or cycle-dependent crack growth phenomena. A modified J-integral is developed to demonstrate how the phasefield approach can be used to generate Paris-law type crack growth rates. In order to model more general crack versus applied loading behavior that are not fit by a single simple Paris-law, steady-state finite element calculations are performed to calibrate fits of the phase-field model to measure crack growth rates found in da/dN versus ∆Kcurves. Transient time- or cycle-dependent calculations are performed and compared to experimental measurements on samples where crack turning is induced by the presence of a hole in the vicinity of the crack. A three-dimensional example with turning of the crack front is also computed illustrating the capabilities of the phase-field approach for complex crack paths. The phase-field approach to fracture replaces sharp crack surfaces with a diffuse fracture zone that represents the traction-free crack faces. The diffuse zone is characterized by a length scale that appears prominently in the governing partial differential equation that governs the evolution of the phase-field variable. Within numerical calculations the diffuse fracture zone must be resolved with a sufficient number of degrees of freedom in order to obtain accurate solutions. Additionally, all material fracture problems possess a physical process zone length scale that scales with the ratio of the fracture energy to the square of the process zone strength. For the vast majority of problems governed by linear elastic fracture mechanics this physical process zone length scale is small, for example it is on the order of microns for aluminum alloys. The most commonly used formulation of the phase-field approach to fracture ties the diffuse phase-field crack length scale to the physical process zone length scale. This presents a challenge for extending such models to large structures that may be on the order of meters in size due to the prohibitive meshing requirements for the phase-field crack length scale. This is especially the case for three-dimensional problems. To address this problem, we present a new form of the phase-field degradation function that allows for a decoupling of the phase-field length scale from the physical process zone length scale. The behavior and limitations of this new formulation are discussed and illustrated with a series of numerical test cases.Item Fold-related brittle structures and associated strain in a limestone bed of the Carmel Formation, San Rafael Swell, Utah(2015-12) Laciano, Peter Joseph; Marrett, Randall; Cloos, Mark; Ukar, EstibalitzThe San Rafael Swell (SRS) is a basement-cored Laramide uplift located in central-eastern Utah. The SRS is bounded on the east by a 70 km long monocline, a fault-propagation fold, with excellent exposure of sedimentary strata including the Carmel Formation. This monocline is an ideal natural laboratory for studying brittle deformation associated with folding. Qualitative and quantitative observations for brittle structures in a limestone bed near the base of the Carmel Fm. were made in a wide range of bedding dip, curvature, and fold domains. Kinematic data was collected for 2942 structures (1865 veins, 746 stylolites, 314 faults) in 30 locations in order to calculate principal directions of strain. Additionally, data was collected along 71 scanlines at 19 of those locations in order to estimate structure intensities and strain magnitudes. Dekameter-displacement thrust faults, acting as ramps between inferred layer-parallel faults, accommodate orders of magnitude more strain than all other observed brittle structures. These faults are only found in segments of the monocline where bedding dip is high, but curvature is low, which provides strong evidence that limb rotation more strongly controls strain magnitudes than layer bending in the SRS. The trishear model effectively predicts SRS monocline geometry, specifically observed limb thickening, broad, curved hinges, and progressively rotating limb. This is likely due to the dominance of thick, homogeneous rock packages, such as the Navajo Sandstone, in the SRS monocline. In contrast, strain localization within the Carmel Fm. is poorly predicted by trishear: there is strong evidence of flexural slip, and folding induced structure orientations and calculated principal strain directions remain consistent relative to bedding. These strain directions are inconsistent with trishear forward models produced by workers such as Zuluaga et al. (2014) that do not stay consistent relative to bedding. These divergences are likely due to the fact that trishear is a kinematic model that assumes rock homogeneity, while the Carmel Fm. is stratigraphically and mechanically heterogeneous. Because this heterogeneity appears to have a strong effect on strain localization, kink band models likely better estimate strain localization in the Carmel limestone bed as well as other layers in folded heterogeneous strata. The monocline’s interpreted transition from layer-parallel shortening to extension at the steepest locations in the monocline, and thus at most advanced stage of folding, enabled estimation of the dip of the basement fault beneath the SRS as ~30°. This shallow dip contrasts with the steep dip (~60°) assumed for the SRS by Zuluaga et al. (2014) and observed in the Kaibab uplift (Huntoon and Sears, 1975; Tindall, 2000), but is consistent with a recent estimation of 20-40° for the SRS by Davis and Bump (2009) using trishear modeling.Item A fracture mechanics approach to accelerated life testing for cathodic delamination at polymer/metal interfaces(2013-05) Mauchien, Thomas Kevin; Liechti, K. M.This work presents a fracture mechanics analysis of the cathodic delamination problem for the polyurethane/titanium and polyurea/steel interfaces. The nonlinear behavior of both polymers was investigated. The recent Marlow model was used to define the strain energy function of the polymers. Viscoelastic effects of the polyurea were also studied. The Marlow model was associated with a nine-term Prony series. This model was seen to represent experimental data relatively well for a wide range of strain rates both in tension and compression. The driving force for delamination, the strain energy release rate G, is presented for both interfaces. Cathodic delamination data for several temperatures are presented as crack growth rate as a function of crack driving force. The approach recognizes that both temperature and stress can be used as accelerated life testing parameters.Item Fracture of high-strength bars in concrete frame members under earthquake loads(2018-09-14) Sokoli, Drit; Engelhardt, Michael D.; Ghannoum, Wassim M.; Hrynyk, Trevor D; Jirsa, Jamos O; Williamson, Eric BFracture of longitudinal bars due to high-strain low-cycle fatigue is a critical failure mode in seismically detailed reinforced concrete frame members because it can lead to rapid strength loss and structural instability. The issue has recently attracted attention due to a national effort aimed at introducing high-strength reinforcing bars (HSRB) with yield strengths of 80 and 100 ksi in concrete construction. The HSRB being produced in the United States possess varying post-yield mechanical properties, such as the tensile-to-yield strength ratio, uniform or fracture elongations, as well as low-cycle fatigue life. The behavior of Special Moment Frame (SMF) members with different types of HSRB subjected to large inelastic demands up to bar fracture is investigated through laboratory testing and analytical examination. Laboratory tests were performed to identify any major issues in the performance of HSRB in concrete members. More specifically, the work aimed to assess the influence of the tensile-to-yield strength (T/Y) ratio, fracture elongation, and shape of stress strain curve of HSRB on the behavior of seismically detailed concrete columns. Four specimens were tested under constant axial load and reverse cyclic lateral loading of increasing amplitudes until fracture of longitudinal bars. Three columns were reinforced with grade 100 bars sourced from different manufacturers and therefore having different post-yield mechanical properties. The fourth column was reinforced with conventional grade 60 ASTM A706 (2016) bars. Concrete columns reinforced with HSRB reached similar lateral drift levels as the specimen reinforced with grade 60 bars before significant loss in lateral strength. A computational framework based on fiber-section elements and mechanics-based behavioral models is proposed to accurately estimate both member-level deformations and strain demands in longitudinal bars and the concrete surrounding them within the plastic hinge regions of frame members. Particularly, the effects of the mechanical properties and steel grade of reinforcing bars on their strain demands are quantified experimentally and estimated by the proposed framework. The strain demands derived through the proposed analytical framework were used to track the damage progress in longitudinal bars that lead to buckling and fracture. A buckling initiation model is proposed that accounts for the mechanical properties of the reinforcing bars, as well as the loading history the bars and the surrounding concrete experience prior to buckling. Material specific bar fatigue relations calibrated through material test results are used to predict the number of half-cycle to bar fracture based on accumulation of strain demands prior and after buckling if it occurs.Item Fracture, friction and granular simulation(2009-05) Yang, Zhiping, 1979-; Marder, Michael P., 1960-This thesis contains three separate yet closely related topics: fracture, friction and simulation of mechanical response of a confined granular medium. The first two are experimental investigations and the last one is a numerical study. In the fracture part, I will describe how to break a piece of silicon in a controlled way such that the atomic nature of the fracture process can be revealed in a macroscopic experiment. In the friction part, I will present another experiment using almost exactly the same setup as for the low temperature fracture experiment to study the properties of static friction and explore ideas concerning the origin of friction. In the last part, I will construct a confined granular packing and study how pulses and continuous waves propagate through it. All these three topics are relevant to geophysical science. I sincerely hope that my study can ignite some fresh thinking in that area and help other researchers to design models that can make more precise earthquake predictions.Item Geostatistical characterization of naturally fractured reservoirs(2004-05-22) Liu, Xiaohuan; Srinivasan, Sanjaynot available